The compounds ##EQU1## wherein each of R.sub.1 and R.sub.2 is H or alkyl of 1 to 5 carbon atoms, a cycloalkyl group of 5 to 10 carbon atoms or R.sub.1 and R.sub.2 bonded together through two adjacent carbon atoms form a saturated cycloaliphatic ring of 5 to 12 carbon atoms, are prepared by step (1) reacting acrylonitrile with isobutyraldehyde in the liquid phase to form 2,2-dimethyl-4-cyanobutyraldehyde and step (2) by reacting 2,2-dimethyl-4-cyanobutyraldehyde with a carbonyl compound of the general formula ##EQU2## wherein R.sub.1 and R.sub.2 are as defined above, wherein step (a) and step (b) are undertaken in the presence of a catalyst which is piperidine or a primary amine and/or a Schiff base and in the presence of an acid compound.